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Study On The Graphene Mode-locked All-solid-state Ultrafast Lasers

Posted on:2013-01-16Degree:DoctorType:Dissertation
Country:ChinaCandidate:J L XuFull Text:PDF
GTID:1118330374980694Subject:Condensed matter physics
Abstract/Summary:PDF Full Text Request
Because of the excellent properties in time and frequency domains, ultrafast pulses have received widespread interests. The significant applications of ultrafast pulses in science, industry, military, environment, energy sources, and communication have improved human life. The passively mode-locked lasers by incorporation of semiconductor saturable absorber mirrors (SESAM) have good performance in ultrafast pulse generation, but the fabrication of SESAM needs complex epitaxial growth techniques such as metal-organic chemical-vaporphase deposition (MOCVD) or molecular beam epitaxy (MBE), leading to a high cost. Moreover, traditional SESAM has narrow absorption wavelength range, lying inside the Vis and NIR region. Therefore, it is crucial to look for a new saturable absorber, which can overcome the shortcoming of SESAM and have better properties. Recently, it has been reported that the graphene, a monolayer of hexagonally arrayed sp2-bonded carbon atoms, have particular capability in nonlinear saturable absorption. In the energy band structure of graphene, there are two carrier relaxation dynamics after photoexcitation, i.e., a fast relaxation time (in10-170fs range) which is interpreted to intraband carrier scattering and following a slow response time (in0.4-1.7ps range) attributed to thermalization processes and interband carrier recombination. The fast response can drive a fast modulation on resonator loss, which is crucial for stabilizing femtosecond laser. The slow one is important for generating picosecond or longer pulses and starting femtosecond laser. The relative investigation has become one of the most active fields in laser research. In this dissertation, we have theoretically and experimentally studied the performance of graphene in all-solid-state pulsed lasers. Firstly, the physical and optical properties of graphene have been systematic investigated. In addition, by using Nd:GdVO4crystal as gain medium, graphene Q-switched and mode-locked operations at1.06and1.34μm were exploited with the realization of stable nanosecond and picosecond pulses. Based on dispersion compensation and optimum resonator design, the femtosecond mode-locked Yb:YGW laser was also achieved. These results revealed that graphene have many attractive properties such as superior electronic transport, strong absorption, controllable modulation depth, small insertion loss, easy mass fabrication, and low cost.1. The structures of lattice and band have been studied together with the corresponding electronic properties. The principles of linear absorption, saturable absorption, and ultrafast relaxation dynamics have been analyzed. The saturation fluence and intensity in different wavelengths have been calculated. We also studied the mechanism of the high thermal conductivity of graphene. The graphene saturable absorber mirrors (SAM) at1.06μm and1.34μm have been fabricated by spin-coating the large-scale graphene sheets on the reflective mirrors. The transmission spectrum of the graphene SAM showed a wideband stable saturable absorption. From it, the layer number of the graphene sheets was calculated to be in the region of2-10.(Chapter2)2. The graphene operates as a fast saturable absorber for the formation of the graphene Q-switched pulse. The dynamic influence of the absorption property on the building of the Q-switched pulses has been theoretically investigated. By using the graphene SAM in the1.06μm Nd:GdVO4laser, stable Q-switching operation was realized. The average output power was2.3W, which is the maximum among the reported graphene Q-switched lasers to our best knowledge. The corresponding pulse width was105ns, repetition rate being704kHz. So the single pulse energy was calculated to be3.2μJ. The optical-optical conversion slope efficiencies were as high as35%and37%, respectively. Moreover, we firstly realized the1.34μm Q-switched laser with graphene SAM. The output beam quality was very close to single transverse mode. The pulse widths, repetition rates, pulse energy under different pump power and output coupler transmission have been measured. With the transmission of3%, the obtained average power was260mW, pulse width being450ns, repetition rate being52kHz, pulse energy being2.5μJ. The optical-optical conversion and slope efficiencies were13.5%and21%.(Chapter3)3. The astigmatism and crystal thermal effect on cavity stability zone has been systematic studied. With optimum resonator design, the stable graphene continuous-wave mode-locked Nd:GdVO4laser has been realized. The minimum pulse duration was16ps. The center wavelength was measured to be at1065nm, with a FWHM of0.58nm, giving a time-bandwidth product of2.455. The large values should be attributed to the large normal dispersion of atomic-layer graphene correlating with the strong electron-photon interaction in the2D lattice. The corresponding average output power and pulse energy were360mW and8.4nJ, respectively, producing an optical-optical efficiency of18.9%and a slope efficiency of22.5%. The ratio of the output power between graphene mode-locking and continuous wave operations reached a high value of0.92, indicating a low insertion loss of the graphene sheets. When the graphene SAM was replaced by a SESAM with the transmission of4%, the pulse duration and average power were measured to be14ps and50mW, corresponding to1.2nJ of pulse energy,3.3%of slope efficiency, and2.6%of optical-optical efficiency. The promising results illuminated that compared with traditional SESAM, our graphene SAM is highly efficient to mode lock a solid-state laser.(Chapter4)4. We firstly demonstrated that the use of graphene as saturable absorber could provide efficient mode-locking modulation in1.3μm neodymium bulk laser. With a Nd:GdVO4crystal, an average power of1.29W was achieved with the pulse duration of11ps and the pulse energy of13nJ. The average power is, to our best knowledge, the highest value ever obtained from graphene mode-locked lasers, and the corresponding optical-optical efficiency of23%is the best result among1.3-μm neodymium mode-locked lasers. The M2beam quality factor of the mode-locked beam reached1.1and1.0in the horizontal and longitudinal planes. We also have studied the performance of SESAM in an optimum resonator. Average output power of2.2W was produced, corresponding to a slope efficiency of17%. The pulse duration was3.3ps, which is the best performance among1.3-μm mode-locked neodymium-doped lasers. These results clearly indicate the dominance of the graphene saturable absorber in high-efficiency and low-cost1.3μm mode-locked lasers.(Chapter5)5. The nonlinear interaction between femtosecond pulses and medium have been theoretically discussed, with the simulation of the influence of the dispersion, chirp, Kerr lens effect, and self phase modulation on the femtosecond pulses. The principle of soliton mode-locking has been also stated in particular. In addition, we experimental realized the soliton mode-locked Yb:KGW laser with graphene SAM and two GTI mirrors. Near-transform-limited428fs pulses were yielded at1031.1nm with an output power of504mW, corresponding to the time-bandwidth product of0.335, the slope efficiency of27%, and the peak power of13.8kW. Both of the output power ratio and the slope efficiency ratio of mode locking to free running reached93%. The M2beam quality factor of the mode-locked beam was1.6and1.4in the horizontal and longitudinal planes. The reason of the pulse splitting under high pump power has also been stated.(Chapter6)The main innovations of this dissertation are as follows1. We have firstly fabricated fully and partially reflective graphene saturable absorber mirrors at1.0and1.3μm, which could be used as the resonator mirrors to lower the insertion loss and improve the laser efficiency.2. The mechanism of Q-switching, picosecond mode-locking and femtosecond mode-locking with graphene as saturable absorber has been theoretically investigated. The guideline of cavity design and optimization has been also systemically stated.3. The1.06μm graphene Q-switched laser yielded2.3W average power, which is the highest value in reported graphene Q-switched lasers. The efficiency of output power conversion from continuous-wave to Q-switching operations was92%, which is higher than any reported passively-switched lasers.4. The performance of graphene in1.34μm Q-switched and mode-locked lasers was firstly investigated. The average power of1.29W was the highest value ever obtained from graphene mode-locked lasers, and the corresponding optical-optical efficiency of23%was the best result among1.3-μm neodymium mode-locked lasers.5. We have realized graphene femtosecond mode-locked bulk laser.428fs pulses were yielded at1031.1nm with a slope efficiency of27%and a peak power of13.8kW. The average power was504mW, which is much higher than that of graphene mode-locked fiber lasers.
Keywords/Search Tags:all-solid-state, graphene, Q-switched laser, mode-locked laser, soliton, ultrafast laser
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